I'm betting when you unplug it you still see "xfinity" but probably not as strong a signal. They're probably all broadcasting the same SSID, just like I only see 1 SSID in my house even though I have 2 WAP's because I set them the same for ease of roaming through the house.
There are some cool tools you can load on your phone too that would let you see each individual WAP signal, unplug your modem and watch to see if one of them disappear.